The invention relates to a method for operating a motor vehicle. A prerequisite is that the motor vehicle has a primary drive for rotating a shaft, for example an internal combustion engine with a crank shaft. The primary drive in this case is coupled with ground contact elements, in particular wheels of the motor vehicle, via an intermediate device. The intermediate device includes a clutch and as the case may be a transmission, wherein the clutch can be integrated into the transmission. A prerequisite is an adjustable intermediate device, in particular an adjustable clutch, i.e. the torque which is transferred via the intermediate device is to be changeable by varying a regulating variable (which in the example more or less directly relates to the clutch) so that a defined rotational speed of the primary drive can be set. In the example, such a clutch can be caused to assume intermediate states between a state of being decoupled and a state in which a maximal torque is transferred, the clutch is completely engaged.
It is known to detect during the drive of such a motor vehicle whether the movement of the motor vehicle occurs in coasting mode, i.e., whether the force flow proceeds from the wheels or other ground contact elements such as a chain, to the (crank)shaft and not vise versa as in the traction mode. Such a trailing throttle phase can for example be detected by way of the position of the gas pedal: in particular when the driver no longer actuates the gas pedal no active forward drive is generated any more by the primary drive (for example internal combustion engine). There is also the so called trailing throttle fuel cutoff: in trailing throttle mode the primary drive is only supplied with the kinetic energy which is inputted via the ground contact units by the clutch, in the example of a motor vehicle with internal combustion engine and transmission the fuel supply to the internal combustion engine is interrupted which saves fuel, the clutch is not engaged, the rotational speed of the crank shaft corresponds to the rotational speed on the transmission side and is thus proportional to the drive output rotational speed of the intermediate device. Small differences in rotational speed result due to a principally possible micro slip which for reasons of simplicity is disregarded in the following description. Due to the fact that the shaft of the primary drive still rotates with sufficient rotational speed an acceleration can be provided again relatively quickly when there is a corresponding demand by the driver for example via the gas pedal.
The trailing throttle fuel cutoff also involves that when exceeding a threshold value for the rotational speed of the primary drive shaft, supply with primary energy (in the case of an internal combustion engine with fuel) is initiated again in order to not allowing the rotational speed of the primary drive to become too low. At this threshold rotational speed the clutch is disengaged.
During the trailing throttle fuel cutoff the primary drive has a decelerating effect on the drive due to internal losses (for example compression in the internal combustion engine, mechanical losses etc.), wherein the kinetic energy which is inputted via the drive is predominantly converted into non-usable energy forms (for example heat). In the following, this no longer usable energy is referred to as loss energy. It would be desirable to be able to convert at least a portion of this energy into electric energy, for example via an electric machine.
As alternative to the trailing throttle fuel cutoff it is also known to disengage the clutch during trailing throttle mode and to operate the primary drive in neutral mode. This prevents that the primary drive absorbs kinetic energy. In addition, no renewed starting process is required for the primary drive in case of acceleration. A disadvantage is here the consumption of primary energy (for example fuel consumption when using an internal combustion engine as primary drive) for the neutral mode.
It is also possible to disengage the clutch during trailing throttle mode and to turn off the primary drive. As a result, neither primary energy nor kinetic energy is absorbed by the primary drive. This has the disadvantage that the primary drive has to be restarted again when accelerating.
It would thus be desirable to find a compromise so as to minimize the disadvantages of the mentioned possibilities of a trailing throttle mode.